The present invention relates to a process for producing electrical energy from natural gas using a solid oxide fuel cell.
A fuel cell is an electrochemical cell which can continuously convert the chemical energy of a fuel and an oxidant to electrical energy by a process involving an invariant electrode-electrolyte system. Here the expression fuel cell is also used to refer to a multiplicity of cells, which can be arranged in series or in parallel.
A solid oxide fuel cell is a fuel cell comprising an anode side and a cathode side separated from each other by means of a solid electrolyte. The solid electrolyte is for example a mixture of yttria and zirconia. Charge transfer through the electrolyte from the cathode to the anode is done by oxygen ions.
The overall cathode reaction of a solid oxide fuel cell is 1/2(a+b)O2+2(a+b)exe2x88x92xe2x86x92(a+b)O2xe2x88x92; and the overall anode reaction is aH2+bCO+(a+b)O2xe2x88x92xe2x86x92aH2O+bCO2+2(a+b)exe2x88x92.
The anode off-gas thus comprises carbon dioxide and water.
Applicant is particularly interested in operating the fuel cell near a well which produces hydrocarbon fluids from an underground reservoir, this can be a gas well or an oil well which also produces associated gas. In both cases a methane-containing gas at high pressure (25-50 MPa) is available. Carbon dioxide obtained as effluent from the process is stored in a receptacle, which can be an underground reservoir. To this end the carbon dioxide has to be compressed to a pressure that allows injecting the carbon dioxide in the underground reservoir. The underground reservoir can be the reservoir from which the hydrocarbon fluids are recovered or an aquafier layer. Thus there are no carbon dioxide emissions.
It is known from European patent specification No. 482.222 to generate electricity from high pressure natural gas using a solid oxide fuel cell. The known process comprises the steps of
(a) supplying oxidant to the cathode side of the fuel cell;
(b) converting at the anode side of the fuel cell the natural gas to hydrogen and carbon monoxide and allowing the cathode and anode reactions to take place to produce a potential difference between anode and cathode wherein an anode off-gas is produced which comprises water and carbon dioxide;
(c) removing oxygen-depleted oxidant from the outlet of the cathode side and removing the anode off-gas from the outlet of the anode side;
(d) feeding the anode off-gas from the outlet of the anode side of the fuel cell to an afterburner installation;
(e) partially condensing the anode off-gas and removing water from the anode off-gas to produce a stream rich in carbon dioxide;
(f) compressing stream rich in carbon dioxide to a predetermined pressure;
(g) cooling the compressed stream rich in carbon dioxide at least partly by indirect heat exchange with the natural gas stream which is supplied to the fuel cell to obtain an at least partly liquefied stream rich in carbon dioxide;
(h) separating uncondensable gas from the at least partly liquefied stream rich in carbon dioxide; and
(i) storing the at least partly liquefied stream rich in carbon dioxide in a receptacle.
In the process disclosed in European patent specification No. 482.222 a conventional afterburner installation is used which is a high temperature oxidation process in which a substantial amount of nitrogen is added to the anode off-gas.
It is an object of the present invention to provide a improved process for producing electricity from natural gas using a solid oxide fuel cell which is provided with an afterburner installation in which the addition of nitrogen to the anode off-gas is minimal or eliminated.
In the process according to the present invention a ceramic afterburner installation is used in which unburned carbon monoxide and hydrogen are combusted without addition of a substantial amount of nitrogen to the anode off-gas.
There is provided a process of generating electricity from natural gas using a solid oxide fuel cell comprising the steps of.
(a) converting at the anode side of the fuel cell the natural gas to hydrogen and carbon monoxide and allowing the cathode and anode reactions to take place to produce a potential difference between anode and cathode wherein an anode off-gas is produced which comprises water and carbon dioxide;
(b) removing oxygen-depleted oxidant from the outlet of the cathode side and removing the anode off-gas from the outlet of the anode side;
(c) feeding the anode off-gas from the outlet of the anode side of the fuel cell to an afterburner installation;
(d) partially condensing the anode off-gas and removing water from the anode off-gas to produce a stream rich in carbon dioxide;
(e) compressing the stream rich in carbon dioxide to a predetermined pressure;
(f) cooling the compressed stream rich in carbon dioxide at least partly by indirect heat exchange with the natural gas stream which is supplied to the fuel cell to obtain an at least partly liquefied stream rich in carbon dioxide;
(g) separating uncondensable gas from the at least partly liquefied stream rich in carbon dioxide; and
(h) injecting the at least partly liquefied stream rich in carbon dioxide into a receptacle;
characterised in that the step (c) of feeding the anode off-gas to the afterburner installation comprises feeding the anode off-gas to a ceramic afterburner installation in which unburned carbon monoxide and hydrogen are combusted without addition of a substantial amount of nitrogen to the anode off-gas.